The present invention relates to an image sensing apparatus provided with an image sensor for generating an electrical signal corresponding to an amount of an incident light and particularly to an image sensing apparatus and an image sensing method using an image sensor having, as photoelectric conversion characteristics, a linear characteristic area where the electrical signal is linearly converted in relation to the amount of the incident light and a logarithmic characteristic area where the electrical signal is linearly converted in relation to the amount of the incident light (a linear characteristic operation and a logarithmic characteristic operation can be switched).
There have been known image sensors (also called “LOG sensor”) in which a logarithmic converting circuit provided with a MOSFET is added to a solid-state image sensing device in which photoelectric conversion elements such as photodiodes are arranged in matrix, a subthreshold characteristic of the MOSFET is taken advantage of to make an output characteristic of the solid-state sensing device such that the electrical signal is logarithmically converted in relation to the amount of the incident light. Some of such image sensors are known to be such that the original output characteristic of the solid-state imaging sensing device, i.e. the linear operative state in which the electrical signal is linearly converted in relation to the amount of the incident light, and the aforementioned logarithmic operative state can be switched.
For example, Japanese Unexamined Patent Publication No. 2002-77733 discloses an image sensing apparatus in which a linear operative state can be automatically switched to a logarithmic operative state and switching points of the respective pixels are set to be equal by giving a specified reset voltage to a MOSFET. Japanese Unexamined Patent Publication No. 2002-300476 also discloses an image sensing apparatus in which a linear operative state can be automatically switched to a logarithmic operative state and a potential state of a MOSFET can be adjusted by adjusting a reset time of the MOSFET.
Since an output proportional to an amount of charges generated by the photoelectric conversion elements can be obtained in the case of using an image sensor in a linear operative state, there are an advantage of being able to obtain a high-contrast (having a high gradation) image signal even if an object has a low luminance and other advantages, whereas there is a disadvantage of a narrower dynamic range. On the other hand, since an output natural-logarithmically converted in relation to an amount of an incident light can be obtained in the case of using the image sensor in a logarithmic operative state, there is an advantage of ensuring a wide dynamic range, whereas there is a disadvantage of a poorer contrast due to the logarithmic compression of an image signal.
The image sensors in the above two publications only disclose that the operative state of the image sensor can be automatically changed from the linear operative state to the logarithmic operative state. However, in view of the aforementioned advantages and the disadvantages of the linear and logarithmic operative states, it is desirable to positively utilize the advantages of the respective operative states to perform an image sensing operation instead of merely automatically switching the operative states. For example, in the case of an automatic exposure control as well, there is a possibility of carrying out an optimal automatic exposure control utilizing the advantages of the respective operative states if the control is carried out in conjunction with the luminance of a subject as a target and the switching point of the image sensor from the linear operative state to the logarithmic operative state.